This invention relates to fibre optic couplers and to their manufacture, and in particular to 1xc3x974 or 2xc3x974 biconical taper fused single mode couplers. The description hereinafter will principally refer to 1xc3x974 couplers but the principles involved are also applicable to 2xc3x974 couplers.
As employed herein, the term 1xc3x974 coupler refers to a fibre optic device which will distribute a signal input through a single input port substantially or nearly equally into four output ports. Coupling ratios which are at least comparable in magnitude, for example in the range 0.15 to 0.35 are presently considered by the fibre optic industry to be xe2x80x9csubstantially or nearly equalxe2x80x9d but of course the achievement of equal ratios of 0.25 is the ultimate object. Such devices are especially useful in fibre optic tree networks.
Fibre optic couplers of 1xc3x974 configuration for single mode applications are presently characterised by a significant degree of wavelength dependence in their principal operating parameters such as coupling ratios and insertion losses. Thus, coupler suppliers generally specify a particular wavelength characteristic for their 1xc3x974 single mode couplers and even so-called xe2x80x9cwavelength flattenedxe2x80x9d 1xc3x974 couplers have approximately flat coupling and insertion loss characteristics only within a single operating band, e.g. 40 nm or 100 nm. It would be desirable to produce a 1xc3x974 single mode coupler which is wideband for the wavelength range 1250-1600 nm, at least in the sense that the coupler is operable in both of the principal transmission wavelength bands for fibre optic communications networks. 1260-1360 and 1430-1580 nm, but more preferably over the whole range 1250-1600 nm.
A number of cross-sectional configurations for the cores of 1xc3x974 biconical taper couplers have been proposed. A simple square array for 4xc3x974 star couplers is described by Mortimore [Electronics Letters (1989), Vol. 25, No. 10, 682], in the context of a particular class of couplers in which the fibres are drawn and fused within a capillary tube which has a lower refractive index then the fibre cladding and which remains an integral component of the coupler. If this array is adapted to a 1xc3x974 splitter coupler, for any given fibre segment which serves as the incident/through fibre, the coupling ratio and insertion loss characteristics with respect to wavelength are similar for the two adjacent fibre segments (which are substantially symmetrically disposed) but quite different for the diagonally opposite fibre segments. Moreover, the cycling of the characteristics with taper ratio and drawn length are quite out of phase, whereby it can be difficult even to control the fusion process to achieve a desired substantially equal power distribution across all four output ports.
Arkwright [Electronics Letters (1991) Vol. 27, No. 19, 1767] describes a five fibre ring in which one fibre is a dedicated input fibre and the other four are output fibres. A proposal for a wholly symmetrical 1xc3x974 coupler configuration is disclosed by Mortimore et al at Electronics Letters (1991), Vol. 27, No. 24, 2252. This configuration entails a hexagonal ring arrangement of fibre segments about a core incident fibre. All fibre segments are of equal diameter and in contact, but only every alternative fibre of the ring is a tap-off fibre: the others are dummy fibres. The result is reported to a quite good broadband response, but a coupler of this kind would be expensive and complex to manufacture.
Australian patent 618108 discloses polygonal section sheaths for a variety of coupler arrays. Sheaths proposed include square and triangular sections, and the patent illustrates or describes 2, 3 or 4 fibre couplers in various arrays.
U.S. Pat. No. 4,798,436 to Mortimore proposes a general technique for adjusting or controlling the characteristics of fibre couplers by varying the relative propagation constants of the fibres of the coupler. For example, one fibre may be differently tapered or different fibres may be selected. Attempts to apply this technique by modifying the fibre segment diagonally opposite the primary or incident fibre segment in the above-mentioned square array, i.e. to compensate for its asymmetric position, have not proven very fruitful. In any event, even if a particular broadband square array could be produced by this approach, its performance would be likely to be too critically dependent on precise fibre position: manufacturing tolerances would be very small.
It is an object of the invention to provide a useful 1xc3x974 or 2xc3x974 fibre optic coupler of a structure which is either wideband in the sense earlier foreshadowed, or is capable, with additional tuning treatment, of exhibiting such a wideband response.
In accordance with a first aspect of the invention, there is provided a fused fibre optic 1xc3x974 or 2xc3x974 coupler in which four optical fibre segments extend longitudinally beside each other in a coupling region in which the fibre segments are at least partially fused together to form an assembly exhibiting a close packed cross-section in which the fibre cores are centered substantially at the comers of a four-sided polygon having a pair of opposite internal acute angles substantially less than 90xc2x0, preferably about or a little less than 60xc2x0.
According to the first aspect of the invention, the invention further provides a fused fibre optic 1xc3x974 or 2xc3x974 coupler in which four optical fibre segments extend longitudinally beside each other in a coupling region in which the fibre segments are at least partially fused together to form an assembly exhibiting a close packed cross-section in which the centre-to-centre spacings of each of the cores of two of the fibre segments from the other three cores are similar.
According to the first aspect of the invention, the invention further provides a fused fibre optic 1xc3x974 or 2xc3x974 coupler in which four optical fibre segments extend longitudinally beside each other in a coupling region in which the fibre segments are at least partially fused together to form an assembly exhibiting a close packed cross-section in which two of the fibre segments are mutually adjacent and lie between the other two. Preferably, said assembly exhibits a twist in the coupling region, for example a twist of at least 45xc2x0. This feature is in marked contrast to prior capillary tube 1xc3x974 fused couplers in which the fibres were laid generally straight and parallel in the tube during the fusion process. In this preferred aspect of the invention, the twist is advantageously applied prior to the fusion and drawing steps and is preferably at least partly applied as a dynamic helical twist.
In accordance with a second aspect of the invention, there is provided a fused fibre optic 1xc3x974 or 2xc3x974 coupler in which four optical fibre segments extend longitudinally beside each other in a coupling region in which the fibre segments are at least partially fused together to form an assembly which is twisted in the coupling region, for example to the extent of at 45xc2x0.
In the second aspect of the invention further provides a method of forming a 1xc3x974 or 2xc3x974 fused fibre optic coupler comprising:
disposing four optical fibre segments so that they extend longitudinally beside each other in a cross-sectional array in which the fibre cores are centered substantially at the corners of a four-sided polygon having a pair of opposite internal acute angles substantially less than 90xc2x0 preferably about or a little less than 60xc2x0;
while maintaining the fibre segments under tension, and with those segments on the shorter diagonal of the polygon preferably under greater tension than the other two segments, applying twist to the fibre segments to form a twisted assembly of the fibre segments, preferably at least half a turn and most preferably 1 to 1xc2xd turns;
heating at least a portion of the twisted assembly and at least partly fusing the fibre segments together while drawing the fibre segments longitudinally, whereby to form a coupling region in which the assembly is twisted.
The twist is preferably at least partly applied as a dynamic helical twist.
Preferably, the aforesaid fibres on the shorter axis of the parallelogram are slightly individually pre-twisted before effecting the aforesaid twisting of the assembly.
In its second aspect, the invention still further provides apparatus for carrying out the aforesaid method.
Preferably, in either the first or second aspect of the invention, one or most preferably both of the fibre segments on the shorter diagonal of the assembly have different propagation constants relative to the other two fibre segments. This may be effected by treating fibre segments of similar diameter in the assembly, or one or most preferably both of the fibre segments on the shorter diagonal may be of a diameter smaller than that of the other fibre segments. The smaller diameters are not necessarily equal. This may be achieved in the manufacturing process by pre-tapering or etching one or most preferably both of the shorter diagonal fibre segments. In this case, of course, in the first aspect of the invention, the internal acute angle of the rhombus will be less than 60xc2x0, and/or the corexe2x80x94core spacings will differ slightly. The degree of pre-taper may vary in the range 0.1 to 30% and will depend on the fusion conditions used during formation of the taper and the temperature profile of the heat source, and will typically be determined by experimentation in a given case.
It is found that the geometry in accordance with the first aspect of the invention is effective in substantially reducing the phase differences between the coupling ratio and insertion loss characteristics for the assembly at a given wavelength, compared with the simple square array previously referred to, and the preferred pre-tapering allows the characteristics at a given wavelength to be made substantially coincident in turning point and phase relationship. By optimising the pre-tapering, it is possible to synchronise substantially equal power distribution in the two main wavelength bands.
Advantageously, when the manufactured assembly has cooled substantially to ambient temperatures in the coupling region, a slight further twist is applied to the assembly. Such a post-twist has been proposed in relation to 1xc3x973 and 1xc3x972 couplers [T A Birks, Applied Optics (1989) Vol. 28, No. 19, Page 4226] and is useful for fine tuning the coupler characteristics. The post-twist may be accumulative or opposite to twist already present in the assembly in accordance with the second aspect of the invention, although it will typically be opposite to such existing twist.
The function of the coupler as a 1xc3x974 or 2xc3x974 coupler is typically determined by terminating three or two of the four fibre segments at one end of the coupler. Preferably, the residual fibre segment, the through fibre, is one of the shorter diagonal fibres. It is found that better results may be obtained by using one of these shorter diagonal fibre segments as the input or launch fibre.